The present invention relates generally to reception and processing of spread spectrum signals in Global Navigation Satellite System (GNSS) receivers. More particularly the invention relates to a receiver according to the preamble of claim 1 and a method according to the preamble of claim 13. The invention also relates to a computer program according to claim 24 and a computer readable medium according to claim 25.
Many examples of GNSSs exist. Presently, the Global Positioning System (GPS; U.S. Government) is the dominant system, however alternative systems are expected to gain increased importance in the future. So far, the Global Orbiting Navigation Satellite System (GLONASS; Russian Federation Ministry of Defense) and the Galileo system (the European programme for global navigation services) constitute the major alternative GNSSs. Various systems also exist for enhancing the coverage, the availability and/or the quality of at least one GNSS in a specific region. The Quasi-Zenith Satellite System (QZSS; Advanced Space Business Corporation in Japan), the Wide Area Augmentation System (WAAS; The U.S. Federal Aviation Administration and the Department of Transportation) and the European Geostationary Navigation Overlay Service (EGNOS; a joint project of the European Space Agency, the European Commission and Eurocontrol—the European Organisation for the Safety of Air Navigation) represent examples of such augmentation systems for GPS, and in the latter case GPS and GLONASS.
The hardware constraints of first generation of GPS receivers were such that these devices processed satellite signals by means of a single channel. In the early designs, the receiver operated sequentially to determine a geographical coordinate based on several satellite signals. M. Weiss describes one example of such a receiver design in PLANS '82—Position Location and Navigation Symposium, Atlantic City, N.J., Dec. 6-9, 1982, Record (A84-12426 02-04). New York, Institute of Electrical and Electronics Engineers, 1982, p. 275-278.
By comparison, all modern GPS receivers employ parallel tracking, for example having 12 channels. This means that the receiver has dedicated hardware to receive 12 channels simultaneously. Typically, this decreases the expected time to identify and acquire the signals from a sufficient number of satellites compared to a single-channel receiver. The parallel receiver also has improved reliability and accuracy.
Furthermore, there exist various forms of hybrid receivers in the form of multiplexing receivers. This type of receiver uses relatively few receiver modules to time division multiplex among the satellites that are currently in view. Hence, the receiver gathers data from one satellite during a first time slot, and then during a second time slot, switches to another satellite to gather more data. Provided that the switching is effected sufficiently fast, the receiver appears to be tracking all of the satellites simultaneously.
Whatever the type of receiver, GNSS navigation can be highly challenging in some radio environments, particularly when the characteristics of these environments are rapidly varying. By design, the signal sources (i.e. the satellites) move across the sky with varying trajectories and elevations depending on the receiver's position relative to the signal sources in question. Moreover, the receiver itself and/or objects between the receiver and one or more of the signal sources may be repositioned, and thus drastically alter the radio conditions. Occasionally, the signals from one or more signal sources may be completely blocked with no prior warning or indication thereof, for example if the receiver passes a corner of a high building. Due to the changeable radio conditions, the set of radio signals based upon which the receiver produces position/time related data must be refreshed repeatedly. However, effecting this updating is not a trivial task, especially not if the available time is short. Namely, in order to include any recently unobscured signals in the navigation solution, a so-called rapid acquisition process with respect to these signals must be completed to determine the data necessary to track them continuously. Failure to re-acquire the tracking data quickly enough may force the receiver to perform conventional re-acquisition or even full acquisition, which consumes significant power and can cause a severe disruption in the production of the position/time related data.